PLANET EARTH

Lecture 2

 

PLANET EARTH

•         third planet in distance from the Sun in the solar system

•          the only planet known to harbor life, and the “home” of human beings.

•         About 71 % of Earth’s surface is covered by water, which is essential to life.

•         rest is land, mostly in the form of continents that rise above the oceans. 

 Origin of the Earth

•         EARLY THEORIES 

 Nebular Hypothesis                                       

•         German philosopher, Kant and French mathematician, Laplace

•         Earth, planets and sun originated from Nebula.

•         Nebula was large cloud of gas and dust. It rotates slowly.

•         Gradually it cooled and contracted and its speed increased.

•         A gaseous ring was separated from nebula

•         Later the ring cooled and took form of a planet

•         On repetition of the process all other planets came into being

•                               The central region, nebula became sun.

v         Planetesimal Hypothesis

•               Chamberlin and Moulton proposed the theory in 1904

•               the solar system originated from matter which was in form of tiny dust grains called     "Planetesimals".

•             The planets evolved from a cloud of planetesimals which revolved around the sun.

•              The planetesimals, the tiny cosmic fragments collided with each other and united  together to form larger bodies which became planets and satellites revolving around the sun.

v    Gaseous Tidal Theory

•         Jeans and Jeffrey proposed the theory in 1925

•         Large star came near the sun.

•         Due to gravitational pull a gaseous tide was raised on the surface of the sun.

•         As the star came nearer, the tide increased in size.

•         Gaseous tide detached when star move away.

•         The shape of the tide was like spindle.

•         It broke into pieces-forming nine planets of the solar system

Modern Theory

•         scientists in later period took up the problem of origin of universe rather than that of just the earth or the planets.

v      Big Bang theory (expanding universe hypothesis)

•         According to cosmologists, the dawn of the universe started with a Big Bang

•         entire matter of the universe was condensed into a cloud of gas and dust

•          after  consecutive expansion and contraction it exploded into the Big Bang

•         The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe.

•         According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly.

•         This rapid expansion caused the Universe to cool and resulted in its present continuously expanding state.

•         According to the most recent measurements and observations, the Big Bang occurred approximately 13.75 billion years ago, which is thus considered the age of the Universe.

•         After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons.

•         protons and neutrons combined to form the first atomic nuclei only a few minutes after the Big Bang,

•          it took thousands of years for electrons to combine with them and create electrically neutral atoms.

•         The first element produced was hydrogen, along with traces of helium and lithium.

•         Giant clouds of these primordial elements would coalesce through gravity to form stars and galaxies, and the heavier elements would be synthesized either within stars or during supernovae.